1. Field of the Invention
The present invention is related to the manufacture of printed circuit boards, and more particularly to a method of masking regions of a plated substrate during a pre-development expose phase of manufacturing.
2. Description of the Prior Art
A printed circuit board (“PCB”) is a rigid or semi-flexible structure designed to have electrical or electromechanical components attached thereto. PCBs may be found in virtually all of the electronic devices with which we come into contact today. The PCB itself typically consists of a non-conductive substrate having a number of patterned conductive interconnections, or traces, formed on one of both sides thereof. The traces permit electrical interconnection of the components attached to the PCB. Typically, both sides of the non-conductive substrate are first plated in copper. The copper is selectively removed, remaining only where electrical interconnection is desired. An insulative material, referred to as a solder mask, is then applied over the patterned copper. Regions of the solder mask are selectively removed where electrical connection to the components is to be made. Regions of solder mask which remain are hardened and form a portion of the finished PCB, electrically isolation copper interconnections thereunder. There are two common techniques for this selective removal of the solder mask.
A first technique is silk-screen printing of an etch resistant material directly over the blanket solder mask layer. The screen is effectively a negative of the final desired pattern of exposed copper. Once deposited, the structure is placed in an etch bath which removes unwanted solder mask material, leaving the desired copper contact points. A variation of this technique is to silk-screen an insulative layer directly onto the substrate and patterned copper.
In a second technique, a photosensitive material (typically a negative, but may be a positive photo sensitive material) is applied to the substrate and patterned copper. A pattern is then optically exposed on the material, either through a photomask or by a position-controlled laser. The light is typically UV. The present invention is particularly relevant to in the masked photo sensitive material process.
In the case of a negative photo sensitive material, the portion of the material that is exposed to UV light becomes cross-linked and relatively insoluble to the developer. The unexposed portion of the material is dissolved by the developer, and those portions of the material exposed to light are etched away, leaving the un-exposed material in place. The remaining material may then be hardened, typically by heating forming the solder mask.
In the case of a positive photo sensitive material, the portion of the material that is exposed to UV light becomes soluble to the developer and the portion of the material that is unexposed remains insoluble to the developer. The exposure, development, and hardening steps are otherwise the same.
The photo sensitive solder mask technique described above has been refined such that it now typically can produce parts with very few defects (i.e., provides very high yield). However, there remains pressure on manufacturers to reduce cost of manufacturing. One significant expense in the manufacturing process is the mask manufacturing. Another is the application of the mask, which requires careful alignment of the mask with reference to landmarks on the substrate. This alignment requires manufacturers to use expensive alignment tools and fixtures during device manufacturing and adds to the production time for such parts.
In order to address the need to reduce the cost of the manufacturing process, print-like processes have been developed for photomasking. For example U.S. Pat. Nos. 6,872,320 and 6,742,884 (each incorporated herein by reference) teach a system and process, respectively, for direct marking of a phase change material onto a substrate in order to produce a photomask. According to these references, a suitable material, such as a stearyl erucamide wax, is maintained in liquid phase over an ink-jet style piezoelectric printhead, and selectively ejected on a droplet-by-droplet basis such that droplets of the wax are deposited in desired locations in a desired pattern on a layer formed over a substrate. The droplets exit the printhead in liquid form, then solidify after impacting the layer, hence the material is referred to as phase-change. We refer to a mask used in the photoresist process described above as a “printed photomask,” and the process used to form the printed photomask as “digital mask printing.”
There are many instances in PCB design where it is necessary to make through-connection from one surface of the PCB to the other. For instance, to reduce the overall size of the PCB, and hence reduce the overall size of the device into which it is installed, both sides of the PCB may contain circuit components. Some of the components on one side of the PCB must be electrically connected to components on the opposite side of the PCB. To do so, an opening is created in the substrate, typically by simply drilling a hole in the desired location. In order to produce an adequate solder connection through the holes, the copper conductive material is allowed to plate the sidewalls of the holes as well as the planar surfaces of the substrate.
Typically, the photo sensitive solder mask material is uniformly spread over the patterned copper and non-conductive substrate. This means that the material ends up not only on the surface of the substrate, but also on the sidewalls of the holes in the substrate made for through-contacts. As mentioned, it is desired that the sidewalls of these holes be conductive, i.e., have copper applied thereto. Therefore, any silk screening or masking for photoresist exposure must be accurately aligned to the holes to prevent expose and hence cross-linking of the resist material.
The problem arises in forming a printed photomask for PCBs having though-holes that the direction of ejection of droplets of phase-change material from the print head is parallel to the sidewall surface. That is, the direction in which the droplets are deposited is such that they do not come to rest on the sidewalls. As it is desired to mask the sidewalls of the through-holes from exposure, the standard printed photomask process is not effective for production of PCBs of this type.
Accordingly, there is a need in the art for a method of forming a printed photomask capable of masking the copper-coated sidewalls of through-holes of a substrate in the process of manufacturing a PCB. The process should be compatible with existing PCB manufacturing processes and equipment so as not to add to the cost and time of production of the final PCB product.